This invention relates, in general, to a means of calibrating a space borne spectrometer and thus improve the operation of the Total Ozone Mapping Spectrometer (TOMS) for measuring stratospheric ozone variations from a satellite.
This improvement removes the effects of spectral changes and stability variations in the calibration source (heretofore man-made) and in the diffuser plate required in the TOMS.
The TOMS basically measures changes in signal levels which are changes in the amount of radiation entering the TOMS and consists of two main subsystems, optics and electronics. As radiation enters the TOMS, the energy passes through the optical subsystem before being processed as an electronic signal in the electronic subsystem. The electronic subsystem can be readily calibrated in space, however, the optical subsystem is not readily characterized and obviously both must be calibrated for the TOMS to provide valid ozone data.
Also, the optical subsystem components in the TOMS are subject to degradation due to contamination and aging. There is no direct way of establishing the amount of degradation. This is usually measured by directing the energy of a known source (man-made) through the optical subsystem and measuring the electronic subsystem response. Theoretically, the TOMS has on-board calibration features which establish the optical subsystem response to known stimuli (source), and in this way, changes in the electronic and optical subsystem response can be segregated. In practice, however, one difficulty is in finding a source which can reliably perform those measurements for years in space without itself degrading and failing. Man-made sources, such as mercury, hydrogen and tungsten lamps, which heretofore have been used to calibrate optical subsystems in the ultraviolet wavelength region, have limited life and are subject to failure. In addition, these man-made sources consume power.
In the past, the sun's energy has been used to calibrate the TOMS with the required diffuser plate directing the energy into the optical subsystem and subsequently into the electronic subsystem, but any change in diffuser plate reflectance was indistinguishable from optical and electronic substances response changes.
Therefore, it is an object of this invention to provide the TOMS with a Solar Reflectance Calibration Assembly (SRCA) which will enable the optical subsystem of the TOMS to be monitored and calibrated over a long time period.